In the TST, the activation energy is the difference of altitude in the potential energy surface [1]} (Fig.1A). The basal axes of this theoretical surface are the popular "reaction coordinates" also called in textbooks "reaction progress", but whose units are not well defined. These coordinates have in particular not the dimension of a spatial length, but the col of the saddle in this landscape is nevertheless crossed by a moving mass point with a velocity defined in the real space (meters/second), thereby mixing theoretical and concrete spaces. Some analogies can however be found between the landscape metaphor and the present theory:
